This is likely caused by an add-on conflict. You may need to restore a backup, remove the offending add-on data from the database, and retry the upgrade. Contact support if you are not sure how to proceed.

Add-ons that cover similar features to ones that are added to 1.2; these may conflict with the core XenForo data. If data conflicts are found, they will need to be resolved in a new add-on release or by removing the add-on before upgrading to 1.2.